1. Field of the Invention
The present invention relates to a composite optical article comprising a base member of glass or a transparent plastic material and a layer of hardenable plastic material formed on a surface of the base member. The invention also relates to a method of manufacturing the composite optical article.
2. Description of the Prior Art
Many lenses, which are typical optical articles, have heretofore been formed of glass. The production cost of aspherical lenses is much higher than that of spherical lenses. In order to solve this shortcoming, a method has been proposed in which a transparent thermoplastic resin is introduced into a mold to form a plastic lens. This method eliminates the necessity for abrasion of products and, thus, is capable of mass-producing lenses at low costs. This method, however, has a disadvantage that, compared with glass, thermoplastic resin suffers from a larger shrinkage when cooled after molding, with a result that the focal length becomes out of order.
In order to eliminate this disadvantage, an improved method has been proposed in which resin is interposed between a metallic mold having a desired optical configuration and a glass base member and hardened to form a composite optical article including a plastic layer with an optical surface formed between the glass base member and the mold. Because the plastic layer is thin, this method is capable of not only assuring small thermal expansion and change of refractive index due to heat but also suppressing the occurrence of strain and thermal shrinkage void.
The conventional method of producing a composite optical article comprises the steps of:
(a) discharging a plastic material onto a plastic layer molding surface of a lens or a plastic base member;
(b) moving a mold having a desired shape towards the plastic layer molding surface to spread the discharged plastic material into a thin film-like plastic layer;
(c) allowing the plastic layer to set; and
(d) removing the mold.
General composite optical articles and methods of manufacture thereof are disclosed in Japanese Unexamined Patent Publication No. 60-56544, No. 60-73816, No. 61-283511, No. 62-288030 and No. 63-110410.
In the conventional methods of producing a composite optical articles described above, if the plastic material is discharged onto the base member at a position offset from the center of the base member or if the deposit of the plastic material is of a poor true-circularity (i.e., not of a circular shape), the plastic material spread by the mold either has its center offset from the center of the base member or forms an oval shape. Accordingly, it is required that the offset of the discharged position of the plastic material and the deviation of the shape of the deposit of the discharged plastic material from the true circular shape be less than several ten .mu.m. It is, however, difficult to assure this precision in view of the fact that a composite optical article requires the plastic material of an amount of from several ten to several hundred mm.sup.3.
In addition, the plastic material tends to be nonuniformly spread due also to uneven wetness of the surfaces of the base member and the mold.
Accordingly, base members and molds of unduly large diameters have been employed so as to avoid the problems discussed above. This has resulted in the production of unduly large and expensive optical articles, each of which is inherently required to have a minimum necessary diameter to provide a necessary optical surface. The optical products incorporating the unduly large composite optical articles are also disadvantageously large-sized.
There has been another method in which an unnecessarily large amount of plastic material is deposited on a base member and then spread by a mold such that a surplus plastic material is caused to protrude out of the mold. This method, however, disadvantageously necessitates an after-treatment for wiping the protruding plastic material off the mold.